In a prior-art burst-switching scheme, a source node sends a burst transfer request to a core node to indicate that a burst of data follows the request. The request indicates the size of the accompanying burst and its destination. Responsive to this burst transfer request, the core node configures a space switch to connect a specific channel in a link on which the burst will be received to any channel in a link directed towards the requested burst destination. The burst follows the burst transfer request after a predetermined time period sufficient to configure the switch and it is expected that, when the burst arrives at the core node, the space switch would have been configured by the core-node controller to accommodate the burst. The core node may fail to schedule the transfer of the burst from input to output at the respective arrival time, and, if the core node does not include a buffer, the burst may be lost. Naturally, the probability of burst loss decreases with the number of channels per link and increases with the number of bufferless core nodes traversed. This switching method is hereinafter called “spatial switching”.
An alternative burst switching method, hereinafter called “temporal switching”, and a mechanism for burst transfer in a composite-star network having optical core nodes is described in Applicant's U.S. patent application Ser. No. 09/750,071, filed on Dec. 29, 2000, and titled “Burst Switching in a High-Capacity Network”, the specification of which is incorporated herein by reference. According to the method, a burst-transfer request is sent to a controller of a selected core node after a burst has been formed at a source node. High network efficiency is realized by pipelining burst scheduling and burst-transfer. The transfer of bursts across the optical core nodes is loss-free.
To realize low-latency temporal switching, Applicant developed a technique according to which burst schedules may be initiated by any of a plurality of bufferless core nodes and distributed to respective edge nodes. A burst size is determined by a core node according to an allocated flow rate of a burst stream to which the burst belongs. The technique is described in applicant's U.S. patent application Ser. No. 10/054,512, filed on Nov. 13, 2001 and titled “Rate-Controlled Optical Burst Switching”, the specification of which is incorporated herein by reference. Burst formation takes place at source nodes, according to information accompanying the burst schedules. An allocated flow rate of a burst stream may be modified according to observed usage of scheduled bursts of a burst stream. A method of control-burst exchange between each of a plurality of edge nodes and each of a plurality of bufferless core nodes enables time coordination, burst scheduling, and loss-free burst switching. Both the load bursts and control bursts are carried by optical channels connecting the edge nodes and the core nodes. The burst descriptors are generated by a master controller of an optical switch in a core node. The switching times of the bursts corresponding to the generated descriptors are scheduled, and the schedules are distributed to the respective edge nodes. The burst-descriptor generation is based on burst-stream flow-rate-allocation defined by the source nodes. A method and an apparatus for allocating an appropriate flow rate to a data stream is described in U.S. Pat. No. 6,580,721, issued to Beshai on Jun. 17, 2003 and titled “Routing and rate control in a universal transfer mode network, the specification of which is incorporated herein by reference.
The burst-switching methods of U.S. patent applications Ser. Nos. 09/750,071 and 10/054,512 require time-coordination where each path from a source edge node to a core node adjacent to a destination edge node is time locked. The network coverage, in terms of the number of edge nodes, is determined primarily by the dimensions of the core nodes.
Extending the network coverage to a global scale may be realized by using a cascade of channel switching and time-slot switching (channel switching establishes a connection from an input port to an output port of a switch and holds the connection for an extended period of time). Extending the network coverage may also be realized by partial use of random-access buffers at selected core channels, as described in Applicant's U.S. patent application titled “Hybrid fine-coarse carrier switching”, filed on Nov. 8, 2002, and assigned Ser. No. 10/290,314, the specification of which is incorporated herein by reference. Such buffers may require optical-to-electrical conversion, electronic storage, and electrical to-optical conversion. The selected core channels are provided with buffers to enable temporal alignment of signals arriving at any core node from several other core nodes. A buffer may be provided at either end of a channel connecting two core nodes. Otherwise, all other ports of the core nodes may be bufferless. Providing such buffers enables the construction of a high-capacity wide-coverage network comprising a large number of core nodes shared by numerous edge nodes. Time-division-multiplexing and burst transfer can coexist in the network disclosed in the aforementioned U.S. patent application Ser. No. 10/290,314 and both are enabled by time-locking each edge node to an adjacent core node (or to a core node reached through channel switching) and by time-alignment at the random-access buffers.
Thus, as described above, extending the coverage of a network based on temporal switching may require an intermediate electronic-switching step either at an intermediate edge node or, preferably, at a random-access buffer placed at selected core channels. It may be desirable, however, to avoid electronic buffering, except at the source edge node and the destination edge node, and explore means for providing fast-switched paths in an entirely optical core in a wide-coverage network.